U.S. Pat. No. 4,850,197, which is assigned to the same assignee as the present application, discloses a vapor compression refrigeration system based on an economizer cycle, such as a screw compressor economizer cycle. The refrigeration system of the aforesaid patent utilizes an economizer heat exchanger which is used in conjunction with an intermediate port of the refrigerant compressor. The economizer heat exchanger enhances a refrigerant cooling cycle by cooling the main refrigerant flow from a receiver to an evaporator. The economizer heat exchanger enhances a refrigerant hot gas heating and/or defrost cycle by adding heat to the heat exchanger during a hot gas heating and/or defrost cycle, to cause the heat exchanger to function as an evaporator.
Stationary refrigeration systems which have an economizer cycle use a flash tank instead of an economizer heat exchanger, with the flash tank having certain advantages over the use of a heat exchanger. For example, the economizer heat exchanger requires a refrigerant charge, thus adding to the total refrigerant charge in the system. A heat exchanger also has an efficiency loss due to the heat exchanger temperature difference across the heat exchange interface. The flash tank, in effect, functions as a perfect heat exchanger, as it has no heat exchange interface, thus providing liquid refrigerant with more subcooling to the expansion valve than a heat exchanger.
Because of these advantages, it would be desirable to be able to use a flash tank in a transport refrigeration system, such as transport refrigeration systems used on trucks, trailers, containers, and the like, to control the temperature of a served cargo space. Prior art flash tanks of which I am aware, however, utilize a suction super-heat valve to control the flow of refrigerant from a refrigerant condenser to the flash tank, and they utilize a float valve to control the flow of refrigerant from the flash tank to an evaporator. A float valve works fine in stationary refrigeration systems where a flash tank is used. A float valve, however, does not perform well and is impractical in a transport refrigeration system, because of the constant movement of liquid refrigerant in the flash tank while the transport refrigeration system is moving with its associated vehicle.